Universal Serial Bus (USB) widely used for data transfer between a host and a peripheral device (i.e., a USB device) In a USB system, the data transfer between the host and the peripheral device is controlled by a host controller, and the communication between the host controller and the host is specified in a host controller interface (HCI), e.g., “eXtensible Host Controller Interface for Universal Serial Bus (xHCI).”
Usually, the host controller executes USB operations to move data between host memory and device endpoints, where each device endpoint corresponds to a uniquely addressable portion of a USB device that is the source or sink of data in a communication flow between the host and the USB device. The host controller may use certain data structures, such as transfer request blocks (TRBs), for USB operations. For example, transfers to and from a device endpoint of a USB device are defined using a transfer descriptor (TD) which includes one or more TRBs. TDs are often managed through one or more transfer rings (i.e., circular queues of TDs) that reside in the host memory, and each transfer ring corresponds to an active device endpoint of a USB device. FIG. 1 depicts an example transfer ring that includes eight TRBs. Command rings and event rings that include a number of TRBs may also be used by the host controller for USB operations.